Far and away the most influential passage in Western philosophy ever written is Plato's discussion of the prisoners of the cave and his abstract presentation of the divided line. For Plato, human beings live in a world of visible and intelligible things. The visible world is what surrounds us: what we see, what we hear, what we experience; this visible world is a world of change and uncertainty. The intelligible world is made up of the unchanging products of human reason: anything arising from reason alone, such as abstract definitions or mathematics, makes up this intelligible world, which is the world of reality. The intelligible world contains the eternal 'Forms' (in Greek, idea ) of things; the visible world is the imperfect and changing manifestation in this world of these unchanging forms. For example, the 'Form' or 'Idea' of a horse is intelligible, abstract, and applies to all horses; this Form never changes, even though horses vary wildly among themselves - the Form of a horse would never change even if every horse in the world were to vanish. An individual horse is a physical, changing object that can easily cease to be a horse (if, for instance, it's dropped out off a fifty story building); the Form of a horse, or 'horseness,' never changes. As a physical object, a horse only makes sense in that it can be referred to the 'Form' or 'Idea' of horseness.

Plato knows these two worlds, the sensible world and the intelligible world, as existing on a line that can be divided in the middle: the lower part of the line consists of the visible world and the upper part of the line makes up the intelligible world. Each half of the line relates to a certain type of knowledge: of the visible world, we can only have opinion (in Greek: doxa); of the intelligible world we achieve "knowledge" (in Greek, epistemŽ). Each of these divisions can also be divided in two. The visible or changing world can be divided into a lower region, "illusion," which is made up of shadows, reflections, paintings, poetry, etc., and an upper region, "belief," which refers to any kind of knowledge of things that change, such as individual horses. "Belief" may be true some or most of the time but occasionally is wrong (since things in the visible world change); belief is practical and may serve as a relatively reliable guide to life but doesn't really involve thinking things out to the point of certainty. The upper region can be divided into, on the lower end, "reason," which is knowledge of things like mathematics but which require that some postulates be accepted without question, and "intelligence," which is the knowledge of the highest and most abstract categories of things, an understanding of the ultimate good.

The Divided Line: The Republic, Book VI

Socrates
"You have to imagine, then, that there are two ruling powers, and that one of them is set over the intellectual world, the other over the visible. I do not say heaven, lest you should fancy that I am playing upon the name. May I suppose that you have this distinction of the visible and intelligible fixed in your mind?"

Glaucon
"I have."

Socrates
"Now take a line which has been cut into two unequal parts and divide each of them again in the same proportion, and suppose the two main divisions to answer, one to the visible and the other to the intelligible, and then compare the subdivisions in respect of their clearness and want of clearness, and you will find that the first section in the sphere of the visible consists of images. And by images I mean, in the first place, shadows, and in the second place, reflections in water and in solid, smooth and polished bodies and the like: Do you understand?"

Glaucon
"Yes, I understand."

Socrates
"Imagine, now, the other section, of which this is only the resemblance, to include the animals which we see, and everything that grows or is made."

Glaucon
"Very good."

Socrates
"Would you not admit that both the sections of this division have different degrees of truth, and that the copy is to the original as the sphere of opinion is to the sphere of knowledge?"

Glaucon
"Most undoubtedly."

Socrates
"Next proceed to consider the manner in which the sphere of the intellectual is to be divided."

Glaucon
"In what manner?"

Socrates
"Thus: There are two subdivisions, in the lower of which the soul uses the figures given by thw former division as images; the enquiry can only be hypothetical, and instead of going upwards to a principle descends to the other end; in the higher of the two, the soul passes out of hypotheses, and goes up to a principle which is above hypotheses, making no use of images as in the former case, but proceeding only in and through the ideas themselves."

Glaucon
"I do not quite understand your meaning."

Socrates
"Then I will try again; you will understand me better when I have made some preliminary remarks. You are awarZ@tudents of geometry, arithmetic, and the kindred sciences assume the odd and the even and teh figures and three kinds of angles and the like in their several branches of science; these are their hypotheses, which they and everybody are supposed to know, and therefore they do not deign to give any account of them either to themselves or others; but they begin with them, and go on until they arrive at last, and in a consistent manner, at their conclusions?"

Glaucon
"Yes, I know."

Socrates
"And do you not know also that although they make use of the visible forms and reason about them, they are thinking not of these, but of the ideas which they resemble; not of the figures which they draw, but of the absolute square and teh absolute diameter, and so on, the forms which they draw or make, and which have shadowsa and reflections in water of their own, are converted by them into images, but they are really seeking to behold the things themselves, which can only be seen with the eye of the mind?"

Glaucon
"That is true."

Socrates
"And of this kind I spoke as the intelligible, although in the search after it the soul is compelled to use hypotheses; not ascending to a first principle, because she is unable to rise above the region of hypothesis, but employing the objects of which the shadows below are resembalcnes in their turn as images, they having in relation to the shadows and reflections of them a greater distinctness, and therefore a higher value."

Glaucon
"I understand that you are speaking of the province of geometry and the sister arts."

Socrates
"And when I speak of the other division of the intelligible, you will understand me to speak of that other sort of knowledge which reason herself attains by the power of dialectic, using the hypotheses not as first principles, but openly as hypotheses, that is to say, as steps and points of departure into a world which is above hypotheses, in order that one may soar beyond them to the first principle of the whole; and clinging to this and then to that which depends on this, by successive steps she descends again without the aid of any sensible object, from ideas through ideas and in ideas one ends. And now, corresponding to these four divisions, let there be four faculties in the soul, intelligence answering to the highest, reason to the second, belief (or conviction) to the third, and perception of shadows or illusion to the last, and let there be a scale of them, and let us suppose that the several faculties have clearness in the same degree that their objects have truth."

Glaucon
"I understand and give my assent, and accept your argument."

Socrates
"And now, I said, let me show in a figure how far our nature is enlightened or unenlightened:, Behold! human beings living in an underground den, which has a mouth open towards the light and reaching all along the den; here they have been from their childhood, and have their legs and necks chained so that they cannot move, and can only see before them, being prevented by the chains from turning round their heads. Above and behind them a fire is blazing at a distance, and between the fire and the prisoners there is a raised way; and you will see, if you look, a low wall built along the way, like the screen which marionette players have in front of them, over which they show the puppets."

Glaucon
"I see."

Socrates
"And do you see, I said, men passing along the wall carrying all sorts of vessels, and statues and figures of animals made of wood and stone and various materials, which appear over the wall? Some of them are talking, others silent."

Glaucon
"You have shown me a strange image, and they are strange prisoners."

Socrates
"Like ourselves, I replied; and they see only their own shadows, or the shadows of one another, which the fire throws on the opposite wall of the cave?"

Glaucon
"True, he said; how could they see anything but the shadows if they were never allowed to move their heads?"

Socrates
"And of the objects which are being carried in like manner they would only see the shadows?"

Glaucon
"Yes, he said."

Socrates
"And if they were able to converse with one another, would they not suppose that they were naming what was actually before them?"
"And suppose further that the prison had an echo which came from the other side, would they not be sure to fancy, when one of the passers-by spoke that the voice which they heard came from the passing shadow?"

Glaucon
"No question, he replied."

Socrates
"To them, I said, the truth would be literally nothing but the shadows of the images."

Glaucon
"That is certain."

Socrates
" And now look again, and see what will naturally follow if the prisoners are released and disabused of their error. At first, when any of them is liberated and compelled suddenly to stand up and turn his neck round and walk and look towards the light, he will suffer sharp pains; the glare will distress him, and he will be unable to see the realities of which in his former state he had seen the shadows; and then conceive some one saying to him, that what he saw before was an illusion, but that now, when he is approaching nearer to being and his eye is turned towards more real existence, he has a clearer vision,, what will be his reply? And you may further imagine that his instructor is pointing And when to the objects as they pass and requiring him to name them, will he not be perplexed? Will he not fancy that the shadows which he formerly saw are truer than the objects which are now shown to him?"

Glaucon
"Far truer."

Socrates
"And if he is compelled to look straight at the light, will he not have a pain in his eyes which will make him turn away to take refuge in the objects of vision which he can see, and which he will conceive to be in reality clearer than the things which are now being shown to him?"

Glaucon
"True, he said."

Socrates
"And suppose once more, that he is reluctantly dragged up a steep and rugged ascent, and held fast until he is forced into the presence of the sun himself, is he not likely to be pained and irritated? When he approaches the light his eyes will be dazzled, and he will not be able to see anything at all of what are now called realities?"

Glaucon
"Not all in a moment, he said."

Socrates
"He will require to grow accustomed to the sight of the upper world. And first he will see the shadows best, next the reflections of men and other objects in the water, and then the objects themselves; then he will gaze upon the light of the moon and the stars and the spangled heaven; and he will see the sky and the stars by night better than the sun or the light of the sun by day?"

Glaucon
"Certainly."

Socrates
"Last of all he will be able to see the sun, and not mere reflections of him in the water, but he will see him in his own proper place, and not in another; and he will contemplate him as he is."

Glaucon
"Certainly."

Socrates
"He will then proceed to argue that this is he who gives the season and the years, and is the guardian of all that is in the visible world, and in a certain way the cause of all things which he and his fellows have been accustomed to behold?"

Glaucon
"Clearly, he said, he would first see the sun and then reason about it."

Socrates
"And when he remembered his old habitation, and the wisdom of the den and his fellow-prisoners, do you not suppose that he would felicitate himself on the change, and pity them?"

Glaucon
"Certainly, he would."<

Socrates
"And if they were in the habit of conferring honors among themselves on those who were quickest to observe the passing shadows and to remark which of them went before, and which followed after, and which were together; and who were therefore best able to draw conclusions as to the future, do you think that he would care for such honors and glories, or envy the possessors of them? Would he not say with Homer:
'Better to be the poor servant of a poor master, and to endure anything, rather than think as they do and live after their manner?'"

Glaucon
"Yes, he said, I think that he would rather suffer anything than entertain these false notions and live in this miserable manner."

">Socrates
"Imagine once more, I said, such a one coming suddenly out of the sun to be replaced in his old situation; would he not be certain to have his eyes full of darkness?"

Glaucon
"To be sure, he said."

Socrates
"And if there were a contest, and he had to compete in measuring the shadows with the prisoners who had never moved out of the den, while his sight was still weak, and before his eyes had become steady (and the time which would be needed to acquire this new habit of sight might be very considerable), would he not be ridiculous? Men would say of him that up he went and down he came without his eyes; and that it was better not even to think of ascending; and if any one tried to loose another and lead him up to the light, let them only catch the offender, and they would put him to death."<

Glaucon
"No question, he said."

Socrates
"This entire allegory, I said, you may now append, dear Glaucon, to the previous argument; the prison-house is the world of sight, the light of the fire is the sun, and you will not misapprehend me if you interpret the journey upwards to be the ascent of the soul into the intellectual world according to my poor belief, which, at your desire, I have expressed, whether rightly or wrongly God knows. But, whether true or false, my opinion is that in the world of knowledge the idea of good appears last of all, and is seen only with an effort; and, when seen, is also inferred to be the universal author of all things beautiful and right, parent of light and of the lord of light in this visible world, and the immediate source of reason and truth in the intellectual; and that this is the power upon which he who would act rationally either in public or private life must have his eye fixed."

Glaucon
"I agree, he said, as far as I am able to understand you."

Socrates
"Moreover, I said, you must not wonder that those who attain to this beatific vision are unwilling to descend to human affairs; for their souls are ever hastening into the upper world where they desire to dwell; which desire of theirs is very natural, if our allegory may be trusted."

Glaucon
"Yes, very natural."

Socrates
"And is there anything surprising in one who passes from divine contemplations to the evil state of man, when they returned to the den they would see much worse than those who had never left it. himself in a ridiculous manner; if, while his eyes are blinking and before he has become accustomed to the surrounding darkness, he is compelled to fight in courts of law, or in other places, about the images or the shadows of images of justice, and is endeavoring to meet the conceptions of those who have never yet seen absolute justice?"

Glaucon<> "Anything but surprising, he replied."

Socrates
"Any one who has common sense will remember that the bewilderments of the eyes are of two kinds, and arise from two causes, either from coming out of the light or from going into the light, which is true of the mind's eye, quite as much as of the bodily eye; and he who remembers this when he sees any one whose vision is perplexed and weak, will not be too ready to laugh; he will first ask whether that soul of man has come out of the brighter life, and is unable to see because unaccustomed to the dark, or having turned from darkness to the day is dazzled by excess of light. And he will count the one happy in his condition and state of being, and he will pity the other; or, if he has a mind to laugh at the soul which comes from below into the light, there will be more reason in this than in the laugh which greets him who returns from above out of the light into the den."

Glaucon
"That, he said, is a very just distinction."

Socrates
"But then, if I am right, certain professors of education must be wrong when they say that they can put a knowledge into the soul which was not there before, like sight into blind eyes?"

Glaucon
"They undoubtedly say this, he replied."

Socrates
"Whereas, our argument shows that the power and capacity of learning exists in the soul already; and that just as the eye was unable to turn from darkness to light without the whole body, so too the instrument of knowledge can only by the movement of the whole soul be turned from the world of becoming into that of being, and learn by degrees to endure the sight of being, and of the brightest and best of being, or in other words, of the good."

Glaucon
"Very true."

Socrates
"And must there not be some art which will effect conversion in the easiest and quickest manner; not implanting the faculty of sight, for that exists already, but has been turned in the wrong direction, and is looking away from the truth?"

Glaucon
"Yes, he said, such an art may be presumed."

Socrates
"And whereas the other so-called virtues of the soul seem to be akin to bodily qualities, for even when they are not originally innate they can be implanted later by habit and exercise, the virtue of wisdom more than anything else contains a divine element which always remains, and by this conversion is rendered useful and profitable; or, on the other hand, hurtful and useless. Did you never observe the narrow intelligence flashing from the keen eye of a clever rogue, how eager he is, how clearly his paltry soul sees the way to his end; he is the reverse of blind, but his keen eye-sight is forced into the service of evil, and he is mischievous in proportion to his cleverness?"

Glaucon
"Very true, he said."

Socrates

Glaucon
"Very likely."

Socrates
"Yes, I said; and there is another thing which is likely, or Neither rather a necessary inference from what has preceded, that neither the uneducated and uninformed of the truth, nor yet those who never make an end of their education, will be able educated ministers of State; not the former, because they have no single aim of duty which is the rule of all their actions, private as well as public; nor the latter, because they will not act at all except upon compulsion, fancying that they are already dwelling apart in the islands of the blest."

Glaucon
"Very true, he replied."

Socrates
"Then, I said, the business of us who are the founders of the State will be to compel the best minds to attain that knowledge which we have already shown to be the greatest of all, they must continue to ascend until they arrive at the good; but when they have ascended and seen enough we must not allow them to do as they do now."

Glaucon
"What do you mean?"

Socrates
"I mean that they remain in the upper world: but this must not be allowed; they must be made to descend again among the prisoners in the den, and partake of their labors and honors, whether they are worth having or not."

Glaucon
"But is not this unjust? he said; ought we to give them a worse life, when they might have a better?"

Socrates
"You have again forgotten, my friend, I said, the intention of the legislator, who did not aim at making any one class in the State happy above the rest; the happiness was to be in the whole State, and he held the citizens together by persuasion and necessity, making them benefactors of the State, and therefore benefactors of one another; to this end he created them, not to please themselves, but to be his instruments in binding up the State."

Glaucon
"True, he said, I had forgotten."

Socrates
"Observe, Glaucon, that there will be no injustice in compelling our philosophers to have a care and providence of others; we shall explain to them that in other States, men of their class are not obliged to share in the toils of politics: and this is reasonable, for they grow up at their own sweet will, and the government would rather not have them. Being self-taught, they cannot be expected to show any gratitude for a culture which they have never received. But we have brought you into the world to be rulers of the hive, kings of yourselves and of the other citizens, and have educated you far better and more perfectly than they have been educated, and you are better able to share in the double duty. That is why each of you, when his turn comes, must go down to the general underground abode, and get the habit of seeing in the dark. When you have acquired the habit, you will see ten thousand times better than the inhabitants of the den, and you will know what the several images are, and what they represent, because you have seen the beautiful and just and good in their truth. And thus our State, which is also yours will be a reality, and not a dream only, and will be administered in a spirit unlike that of other States, in which men fight with one another about shadows only and are distracted in the struggle for power, which in their eyes is a great good. Whereas the truth is that the State in which the rulers are most reluctant to govern is always the best and most quietly governed, and the State in which they are most eager, the worst."

Glaucon
"Quite true, he replied."

Socrates
"And will our pupils, when they hear this, refuse to take their turn at the toils of State, when they are allowed to spend the greater part of their time with one another in the heavenly light?"

Glaucon
"Impossible, he answered; for they are just men, and the commands which we impose upon them are just; there can be no doubt that every one of them will take office as a stern necessity, and not after the fashion of our present rulers of State."

Socrates
"Yes, my friend, I said; and there lies the point. You must contrive for your future rulers another and a better life than that of a ruler, and then you may have a well-ordered State; for only in the State which offers this, will they rule who are truly rich, not in silver and gold, but in virtue and wisdom, which are the true blessings of life. Whereas if they go to the administration of public affairs, poor and hungering after their own private advantage, thinking that hence they are to snatch the chief good, order there can never be; for they will be fighting about office, and the civil and domestic broils which thus arise will be the ruin of the rulers themselves and of the whole State."

Glaucon
"Most true, he replied."

Socrates
"And the only life which looks down upon the life of political ambition is that of true philosophy. Do you know of any other?"

Glaucon
"Indeed, I do not, he said."

Socrates
"And those who govern ought not to be lovers of the task? For, if they are, there will be rival lovers, and they will fight."

Glaucon
"No question."

Socrates
"Who then are those whom we shall compel to be guardians? Surely they will be the men who are wisest about affairs of the state."

Summary